1. Technical Field of the Invention
The present invention relates generally to a fuel injection system for internal combustion engines designed to control a fuel discharge from a fuel injection pump by regulating the quantity of fuel sucked into the fuel injection pump.
2. Background Art
U.S. Pat. No. 4,492,534 (Japanese Patent First Publication No. 59-65523) teaches a fuel injection system which has a metering valve disposed in a fuel feed line between a feed pump and a fuel injection pump and controls an on-off duration in which the metering valve is turned on and off cyclically to regulate the quantity of fuel sucked into the fuel injection pump. The metering valve is switched between a fully opened position and a fully closed position. The ratio of a change in sucked quantity of fuel to a change in on-duration in which the metering valve is opened is, thus, increased as the on-duration is prolonged, which leads to difficulty in controlling the quantity of fuel supplied to the fuel injection pump accurately by changing the on-duration of the metering valve.
Usually, the fuel injection pump sucks the fuel in synchronism with rotation of the engine. An increase in engine speed, thus, causes the length of time the fuel injection pump sucks the fuel to be decreased. Switching between the fully opened position and the fully closed position of the metering valve consumes the response time inherent to the metering valve. Therefore, for instance, when the engine is operating at high speeds and high loads, and it is required to increase the quantity of fuel discharged from the fuel injection pump, it will cause the switching between the fully opened position and the fully closed position of the metering valve to become fast, thus resulting in difficulty in controlling the quantity of fuel sucked into the fuel injection pump if the response rate of the metering valve is low.
This problem may be alleviated by elevating the voltage for energizing the metering valve to increase the response rate thereof. This is, however, impractical because of increase in manufacturing cost.
The adjustment of quantity of fuel supplied to the fuel injection pump may alternatively be achieved by changing an opening area of the metering valve. In typical distributor type fuel injection pumps, the current supplied to a timing control valve is controlled to regulate an opening area of the timing control valve in order to control the fuel injection timing. This technique may be employed to regulate the quantity of fuel sucked into the fuel injection pump.
However, when the engine is operating at low speeds, it will cause the length of time the fuel injection pump sucks the fuel to be increased. Therefore, a ratio .DELTA.q/.DELTA.L of a change in quantity of fuel discharged from the metering valve to a change in opened area of the metering valve, as shown in FIG. 16, increases as the engine speed decreases. For example, during idle modes of engine operation, it becomes difficult to control the quantity of fuel supplied to the fuel injection pump accurately.